Seismic Structure beneath the Vancouver Island Continental Shelf Using Fullwaveform Inversion of Seismic Data

In the structurally complex accretionary wedge near the deformation front of the Northern Cascadia subduction zone, conventional seismic data processing and tomography are inadequate to obtain high-resolution velocity models, given the effects of steep dips and high degree of lateral variations in seismic velocity. Hence we apply frequency domain fullwaveform inversion to marine multichannel seismic reflection data on the continental shelf off Vancouver Island. Our objectives are to obtain detailed seismic velocity structure of the Tofino fore-arc basin and to understand the relationship with underlying accreted terranes such as the Crescent terrane and the Pacific Rim terrane. The results show a prominent low velocity zone in the Tofino basin sediment section. We interpret this to be a result of overpressure in the sediments associated with tectonic shortening and fluid flow. Shallow high velocities were observed in the mid-shelf region where an anticlinal structure was observed. This is interpreted as the shallowest occurrence of the volcanic Crescent terrane. The observed landward increase in seismic velocity indicates the over- consolidation of sediments, associated with the compressional environment at the time of emplacement of Crescent and Pacific Rim terranes.